Since the mediator shouldn't talk directly to the proxy, it should use a notification.
This idea is stated regularly in the forums. It is not true. Mediators CAN talk to proxies; they don't have to but they can. See following taken from the Best Practices document.
Since Mediators will also frequently interact with Proxies, it is common for a Mediator to retrieve and maintain a local reference to frequently accessed Proxies in its constructor. This reduces repetitive retrieveProxy calls to obtain the same reference.